The present invention relates to a battery including an anode active material layer which includes silicon (Si) as an element, and a method of manufacturing the battery.
In recent years, as mobile devices have become more sophisticated and multifunctional, a demand for an increase in the capacity of secondary batteries as power sources for the mobile devices has been made. As a secondary battery which meets the demand, a lithium secondary battery is cited. However, in a currently typical lithium secondary battery which uses lithium cobalt oxide for a cathode and graphite for an anode, its battery capacity has reached the saturation point, so it is extremely difficult to significantly increase its capacity. Therefore, using metal lithium (Li) for an anode has been considered since a long time ago; however, in order to put the anode to practical use, it is necessary to improve lithium precipitation/dissolution efficiency and control dendritic evaporation.
On the other hand, in recent times, a study of an anode with high capacity which uses silicon, tin (Sn) or the like has been conducted vigorously. However, when charge and discharge are repeated, the anode with high capacity is broken into small pieces due to severe expansion and shrinkage of an active material, thereby a current collecting property declines, and the decomposition of an electrolyte solution is accelerated due to an increase in a surface area, so cycle characteristics are extremely poor. Therefore, an anode formed through forming an active material layer on a current collector by a vapor phase method, a liquid phase method, a firing method or the like has been studied (for example, refer to Japanese Unexamined Patent Application Publication No. H8-50922, Japanese Patent No. 2948205 and Japanese Unexamined Patent Application Publication No. H11-135115). The anode can be prevented from being broken into small pieces, compared to a coating type anode formed through applying slurry including a particulate active material, a binder and the like in a related art, and in the anode, a current collector and an active material layer can be formed as one unit. Therefore, the electronic conductivity in the anode is excellent, and higher performance in terms of capacity and cycle lifespan is expected. Moreover, an electrical conductor, a binder and voids which are present in an anode in a related art can be reduced or eliminated, so the anode can be formed into a thin film in essence. Further, it has been reported that the surface of the anode current collector is roughened so as to form microscopic asperities on the surface of the anode active material, thereby excellent characteristics can be obtained (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-83594, S. Fujitani, H. Yagi, K. Sayama, T. Yoshida, H. Tarui, Sanyo Electric Co., Ltd., “The Electrochemical Society 203rd Meeting (Paris, France) Abstract 1152”, (New a-Si Alloy Thin Film Anode with Self Organized Micro Columnar Structure), P. 1152, and “Summary of Autumn Meeting of the Electrochemical Society of Japan” 2002, p. 107).
However, in such an anode, for example, as described in S. Fujitani, H. Yagi, K. Sayama, T. Yoshida, H. Tarui, Sanyo Electric Co., Ltd., “The Electrochemical Society 203rd Meeting (Paris, France) Abstract 1152”, (New a-Si Alloy Thin Film Anode with Self Organized Micro Columnar Structure), P. 1152, in an anode active material layer, individual primary particles which grow long and thin in a thickness direction repeatedly expands and shrinks by charge and discharge, so in the early stages of cycles, relatively good characteristics can be obtained; however, when the cycles are repeated, the anode active material layer may be damaged or fall off a current collector, thereby the characteristics decline.
Such a phenomenon often occurs in the case where the adhesion between primary particles is poor; however, when the adhesion between primary particles is strong, and thereby secondary particles become too large, the secondary particles may fall off according to charge and discharge, or a current collector may be damaged because it is difficult to release a stress by the expansion and shrinkage of the secondary particles. Therefore, it is difficult to obtain sufficient characteristics.